Petrochemical Tank Emissions: Understanding Partial Speciation

No matter how stellar your data management practices are for materials stored or blended in your tanks, you’re likely to come across a substance that you simply cannot know the exact chemical composition for. The most common example is crude oil: any refinery that deals with crude oil must also deal with a handful of unknown chemical properties which makes reporting air emissions difficult.

Even a small fraction of an overlooked chemical in your refined substance can have an enormous impact on the emission calculations you perform later on. Not knowing the full chemical speciation of your products is a common scenario for EH&S professionals in the petrochemical industry. In these cases, partial speciation calculations must be performed.

When to Use Partial Speciation

Partial speciation can be used to determine emissions from a liquid stock – including emissions from the chemicals of interest for which you have weight fractions provided. In other words, you don’t need to know everything about your material, but you do need to have data on hand about the criteria chemicals in your stock.

Whenever you don’t have a full picture of what is in your tank, but you do have information about the most important parts, you should select the partial speciation method.

You’ll need to know the following variables (or have the raw data available to calculate these variables):

The stock's True Vapor Pressure (TVP). This can be extrapolated from other available chemical data (RVP and distillation slope).

The stock's liquid molecular weight.

The stock's vapor molecular weight.

You should at least be able to know these variables about your stock. If not, you’ll either have to perform testing/analysis, improve the quality of EH&S data you get from suppliers, or rely on standardized industry data. The American Petroleum Institute (API) offers tables and guidance documents that provide accepted TVP and molecular weight values for basic liquids like crude oil.

Partial Speciation Method: Where to Find It

Once you have all of the above data prepared and recorded, you can begin calculating emissions under the partial speciation method. The API provides full and detailed instructions on the process, and we recommend using the API documents whenever possible.

The API provides a 12-step process for the partial speciation method, which is available for purchase from the API website. We can't share it with you here due to intellectual property regulations but we believe so strongly in the API's process that we use it ourselves.

It should be noted that the actual equations, tables, and industry constants are the intellectual property of the American Petroleum Institute. If you need to learn the exact values and equations needed to complete the above steps, you should contact the API directly.

Improving Tank Emissions Calculations

The partial speciation method can be a lot of work, both in terms of labor and time. That’s because partial speciation is often the result of having missing or bad data. Using a full speciation method – in which you know the full chemical breakdown of you entire stock rather than just the key chemicals – is preferable for almost all mixtures (the exception being petroleum liquids which need partial speciation). But sometimes partial speciation is unavoidable for certain mixtures used by the petroleum industry.

There are ways to make tank emission tracking a more efficient task. The first and most important one is to automate tank emission calculations as much as possible: the Oil & Gas industry has seen a boom in the number of e-reporting and calculation tools being rolled out. By using a tool with the partial and full speciation methods built in, you’ll dramatically reduce how many hours each week are spent on emissions tracking.

A word of caution, when investigating the tools available on the market, look for one that isn’t a one-trick pony: if a tool can only perform calculations for one operating scenario, or for one type of tank, or only for petroleum liquids and not for chemical mixes, you’ll be selling yourself short. There are solutions and tools available that are comprehensive and able to meet all your business’ needs.

In the chemical industry, not knowing exactly what’s in your tanks is usually a case of being provided inadequate data – either your suppliers don’t know themselves, or have given you approximate data in order to protect their own confidential blends. In either case, it’s your business left shouldering the responsibility of emission reporting. You could use partial speciation, but you’re better off getting all the data you’re entitled to and using the full speciation method instead.

We encourage any business using tanks to do a performance assessment on how they collect material/chemical data. Are you entering it by hand (slow, and prone to errors)? Is it shared via spreadsheets (faster, but cumbersome for multiple materials and revisions)? Is it electronically shared between secure databases (fastest and most accurate option, but requires an initial investment to set up)?

The third option, a shared electronic database, is the ideal option. Once it has been set up between your business and suppliers, data will come in faster, more securely, and the overall quality can be improved by setting the system up to only provide relevant EH&S data for emission reporting so that confidential data is protected. This entire project can become quite involved, so you may find it more helpful to work with someone who has experience designing these shared database systems.

Improving your data management could be the step that increases how often you use the simpler full speciation method for tank emissions, resulting in less time spent doing calculations.

Action Plan

Any business that has to report emissions from tanks, including Oil & Gas and Chemical businesses can maximize operational efficiency by following these steps:

Learn how to properly use the partial speciation method: refer to the basic steps in the API documents for exact equations and industry-standard variables for more complex stock liquids.

Investigate situations in which you've used partial speciation: is it because the complexity of the stock requires it or is it because data is missing or of poor quality? If you find yourself doing partial speciation to compensate for poor data, there's an opportunity to improve.

Look for ways to improve data management and collection. This could mean a total system overhaul or simply cleaning up your existing databases.